<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <script src="https://cdn.bootcdn.net/ajax/libs/vue/2.6.10/vue.js"></script>
</head>
<body>
<div id="app">
    <input type="text" v-model="msg"/>
    <button @click="count++">累加count</button>
    doubleCount:{{doubleCount}}
</div>
<script>
    const vm = new Vue({
        el:"#app",
        data(){
            return{
                msg:"",
                count:2
            };
        },
        computed:{
            doubleCount(){
                return this.count *2;
            }
        },
        watch:{
            msg(newVal,oldVal){
                console.log("发送请求","新增" + newVal,"旧值" + oldVal )
            },
            doubleCount() {
                console.log("重新计算了")
            }
        }
    })
</script>
</body>
</html>